The invention relates to a hydraulic jack, particularly to a two-speed hydraulic jack which ascends at a high speed before a load is encountered and ascends at a lower speed after the load is encountered.
One conventional two-speed jack is shown in U.S. Pat. No. 3,782,689 of Barosko. The jack of Barosko includes a pump, a small-diameter high speed chamber, a low speed chamber and a sliding member disposed between the high and low speed chambers. In a illustrated embodiment, the sliding member acts as a combined valve and seal. It is found that the provision of such a sliding member has complicated the construction of the jack.